This is a scan of the revised and abridged edition which was originally published in 1965.
From Wikipedia: "On The Track Of Unknown Animals is a cryptozoological book by the Belgian-French zoologist Bernard Heuvelmans that was first published in 1955 under the title Sur la Piste des Bêtes Ignorées. The English translation by Richard Garnett was published in 1958 with some updating by the author and with a foreword by Gerald Durrell. A revised and abridged edition was published in 1965, and a further edition in 1995. It is credited with introducing the term cryptozoology and established its author as the "Father of Cryptozoology."
As one reviewer explained, it is a book "about animals that might exist." On The Track Of Unknown Animals cites animals that had only been discovered relatively recently, such as the pygmy chimpanzee, coelacanth, Komodo dragon and giant panda; and those that are believed to have become extinct relatively recently, such as the moa and Tasmanian tiger. A major theme is that these animals were generally known to local peoples, but their stories were dismissed by visiting zoologists, particularly the okapi.
The author then discusses evidence for mystery animals from all over the world including the Mokele-mbembe, sea serpents and the Yeti, with an extensive bibliography. He begins by complaining that "The Press has made such a laughing-stock of the Loch Ness Monster... that no scientific commission has ever dared tackle the problem" and ends with the wish that any new species are not merely slaughtered for trophies: "Have pity on them all, for it is we who are the real monsters.""